(a) Field of the Invention
The present invention relates to an inverter and a liquid crystal display including an inverter.
(b) Description of the Related Art
Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and plasma panel displays (PDPs) and non-emitting displays such liquid crystal displays (LCDs) requiring light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.
The light may be emitted from a light source equipped in the LCD or may be natural light. When using the equipped light source, the total brightness of the LCD screen is usually adjusted by regulating the ratio of on and off durations of the light source or regulating the current through the light source.
A lighting device for an LCD, i.e., a backlight unit usually includes a plurality of fluorescent lamps such as external electrode fluorescent lamps (EEFLs) and cold cathode fluorescent lamps (CCFLs) as light sources, which uniformly transmit the light to the entire front surface of the LC panels from the rear of the LC panels.
Although the CCFLs have been mainly used in the backlight unit for the LCD, the EEFLs have been recently watched due to relatively low cost and simplicity of driving lamps connected in parallel. Since the CCFL has an internal electrode in a glass tube, it needs a separate ballast capacitor. However, since the EEFL has external electrodes in both ends of a glass tube, it needs no separate ballast capacitor. The glass tube included in the EEFL separates the external electrode from the inside of the glass tube to function as a ballast capacitor. Accordingly, in this case, it is relatively easy to drive lamps connected in parallel.
This EEFL must be supplied with the lamp tube voltages having the same magnitude at both ends of the lamps since the EEFL has a symmetrical structure that ballast capacitors are placed in both ends of the glass tube. Moreover, the EEFL must have a difference between the voltages applied at both ends of the glass tube. Accordingly, it is difficult to employ ground type fluorescent lamp driving method that one of electrodes in the lamp is fixed at a predetermined voltage such as a ground. Therefore, the EEFL is adapted to employ floating type fluorescent lamp driving method that voltages having the same magnitude and the different polarities are applied at both ends of the lamp. In other words, phase difference between the voltages is 180°
The backlight unit includes an inverter for driving these lamps. The inverter converts a DC (direct current) input voltage into an AC (alternating current) voltage and applies the AC voltage to the lamps. Then, the inverter controls a light source according to input luminance control voltage to adjust luminance of the lamps.
In order to maintain uniform luminance, it is important to control a tube current flowing through the lamp such that the tube current is constant. However, it is not easy to control the tube current in floating type fluorescent lamp driving method due to no lamp electrode connected to a ground.
Meanwhile, the inverter includes a transformer for converting a low voltage into a high voltage. Since the transformer induces the high voltage having a high frequency, leakage currents occur to flow in the transformer, a transformer cover, the lamp, and so on. However, when the inverter controls the voltage applied to the lamp on the basis of the currents flowing in the inverter without considering the leakage currents, the tube current may be unstable. Accordingly, luminance of the LC panel may not become uniform.